1. Field of the Invention
The human spine is a flexible structure comprised of thirty-three vertebrae. Intervertebral discs separate and cushion adjacent vertebrae. The intervertebral discs act as shock absorbers and allow bending between the vertebrae.
This invention relates to an intervertebral disc prosthesis that allows improved movement and range of motion for the recipient. The intention of this invention is to replace the disc with a device that mimics the geometrical and mechanical characteristics of a normal human intervertebral disc. The invention is a flexure system that acts as a spring. When acted on by any force or movement, the prosthesis of the present invention will react similarly to the reaction of a normal human intervertebral disc.
2. Description of Related Art
There have been devices developed and tested in the past in attempts to design a successful disc replacement. Problems with the development of a successful device include the difficulty in maintaining safety to the spinal cord and its nerve roots and that of the supporting body and head in a variety of postures during normal movement of the arms, legs, and torso.
For example, U.S. Pat. No. 5,827,328 to Butterman discloses an intervertebral prosthetic device that includes a compressible member with at least one spring that is pre-loaded to place the annulus fibrosis under tension and to reproduce the mechanical properties of a natural disc.
U.S. Pat. No. 5,320,644 to Baumgarter discloses a intervertebral disc prosthesis, but fails to disclose, among other things, a disc prosthesis with perimeter openings and a support ball. U.S. Pat. No. 5,895,428 to Berry discloses an implant that has an upper member that pivots and is locked into a lower member. Additionally, U.S. Pat. Nos. 5,556,431; 5,782,832; 5,888,226; 5,865,846; 5,888,223; 5,676,702; 4,932,975; and 5,423,817 generally relate to the area of disc replacement.
Many spine operations involve problems with the intervertebral disc. The goal of spinal surgery is fusion, which is affected by biomedical factors including the load carried by a spinal fixation device and the flexibility of the spine-implant. It has been demonstrated that a boimechanically strong and clinically safe device restores sagittal alignment and stability. Restoration of the load carrying function of the spine is best accomplished by positioning the implant in the middle column of the spine. See Dawson et al., The Spinal Nail: A new implant for Short Segment Anterior Instrumentation of the Thoracolumbar Spine, Spine, 9(4), 1996; the contents of which are incorporated herein by reference.
Further, the designs of most spinal instrumentation systems are predicated upon eliminating motion at the affected levels. See Gurwitz et al., Biomechanical Analysis of Three Surgical Approaches for Lumbar Burst Fractures Using Short Segment Instrumentation, Spine 18:977-982, 1993.
A prior accepted remedy for disc resection has been disc fusion of the vertebra immediately superior to the resected disc space with the vertebra immediately inferior to the resected disc space. While in the past such fusion has had satisfactory results, such results are mixed. For example, the discs adjacent to the fusion site have a greater tendency to fail. This failure is believed by some to be caused by altered stress from altered spine kinematics. An additional reason for the additional disc failure may be the demands on the fusion device.
The disc prosthesis of the present invention has the objective of providing a vertebral disc prosthesis that will perform effectively and efficiently within a patient""s spine over a long period of time, and that will not encourage degeneration of or cause damage to adjacent vertebrae. Further, it is flexible and permits limited motion rather than rigidly constraining its spinal level.
With regard to motion, the disc prosthesis of the present invention permits flexion-extension and lateral bending. These motions are achieved by deformation of the implant rather than articulation between parts as with other implants. Deformation occurs in the flexure system with each slit or slot behaving like a hinge.
The disc prosthesis of the present invention generally is an excellent implantable intervertebral disc prosthesis that comprises a disc member having an upper surface, a lower surface, and a perimeter surface. The disc includes an axis at least one slit defined in the perimeter surface; the slit being of sufficient depth and thickness to provide flexure. The slit terminates in a perimeter opening larger than the slit thickness.
Preferably the disc prosthesis of the current invention is shaped to fit between two vertebrae, and thus may have convex or flat upper or lower surfaces, as well as other appropriate shapes (i.e., oval or kidney shaped). Additionally, the vertebrae may be shaped to better receive the prosthesis of the present invention and thereby provide a more snug fit.
The perimeter opening (i.e., hole) acts as a pivot point and relieves stress. The hole may be circular or non-circular, centered or non-centered on the slit. Preferably, the hole is circular.
In another embodiment of the present invention, the replacement prosthesis, comprises an upper disc having an upper and lower surface and an upper seat defined on the lower surface, a lower disc having an upper and lower surface and a lower seat defined on the upper surface, and a support ball. The support ball engages the seat on the upper disc and the seat on the lower disc when the upper and lower discs are attached to one another and ready for insertion into the spine.
As stated above, the disc prosthesis of the present invention permits flexion-extension and lateral bending for a recipient of the disc by deformation of the disc. Preferably, the disc prosthesis of the present invention comprises a cavity through part of the axis of the disc, with the cavity housing a support ball. The support ball transfers the axial compression load and thereby decreases the deformation caused by axial compression. Additionally, loads that induce flexion-extension or lateral bending of the spine are also major components of spinal loading. Off-center compressive loads will cause the disc of the present invention to flex. With regard to flexion-extension, such loads will close a slit in hinge-like fashion and at the same time push against the support ball. An opposite slit will open because it rotates on the support ball. Because at least two opposite xe2x80x9chingesxe2x80x9d are involved, the angle through which the disc prosthesis of the present invention is increased. Preferably, the angle is doubled. Additionally, the support ball maintains disc prosthesis height. If the prosthesis flattens any, pinching of the nerves may occur which could cause the recipient substantial pain. The support ball is preferably ceramic.